The long-term objective of this project is to understand how the first axons, called pioneers, in the developing nervous system are guided to their targets. The major sub-goals are to (1) identify nematode pioneer neurons, (2) characterize the non-neuronal cells and extracellular matrix which line the paths of the pioneer axons, (3) identify the final target cells of the pioneer neurons, (4) identify the adhesive ligands or other guidance signals made by the pathway and target cells, and (5) determine how the neuronal cytoskeleton uses these cues to steer the axon. The molecules that guide pioneer axons along the nematode epidermis are to be identified in Caenorhabditis elegans by selecting mutants with abnormal axon trajectories. This genetic approach makes few assumptions about the cellular or molecular mechanisms of guidance, and in principle, can reveal even minor components and those for which no suitable in vitro assay, such as neurite extension, exists. The immediate aims of this project are to (1) molecularly clone and sequence the unc-5, unc-6, and unc-40 genes which appear by genetic and anatomical studies to encode a functionally related set of epidermal molecules needed for guiding pioneer axons along the dorsal/ventral coordinate of the nematode body, (2) to identify the genes which encode neuronal receptors for these presumptive guidance cues, and (3) genetically identify the epidermal molecules that guide pioneer axons along the other major coordinate, anterior/posterior, of the body. Knowledge of the molecular structures and biochemical functions of the unc-5, unc-6, and unc-40 genes and their products will contribute to our understanding of the chemical and cellular events underlying axon guidance in nematodes and higher animals.